Supersonic nozzles are well known in the art. Conventional converging-diverging nozzles include a converging section, a throat, and a diverging section. If sufficient pressure is applied to a converging-diverging nozzle, air velocity of the throat will become sonic, and then increase as the air expands at the diverging section to produce a supersonic outlet velocity. The exact exit velocity depends on air pressure, size, and other details of the nozzle design.
Nozzles may be used for a variety of purposes. They may be operated automatically while located a distance from the operator and other people. Other nozzles may be hand held by an operator who directs the exiting flow in order to achieve a particular purpose. One specific use of such nozzles is in combination with a particle blast cleaning apparatus such as that disclosed in U.S. Pat. No. 4,744,181. Such nozzles are typically of the type hand held by an operator, who directs the flow, which is a mixture of transport gas flowing at a supersonic velocity and carbon dioxide pellets carried along by the transport gas. The flow is directed onto an object to be cleaned by particle blast or cryogenic particle blast cleaning methods.
A major problem with the use of supersonic nozzles is the decibel level of the noise emitted by the nozzle during operation. Such noise becomes a critical factor in the acceptability and use of a particular nozzle design when people are required to be nearby. Furthermore, when such nozzles are used in an enclosed area, such as a factory, the reflective surfaces of the area can tend to increase the decibel level experienced at particular locations.
A sound pressure level of 120 dB on the A scale (dBA) has been determined by OSHA to be the threshold level of pain for a human being. However, in occupational situations, OSHA limits the exposure level of a person to such noises to less than 90 dBA for an eight hour time period. Industry and military standards establish this level at 85 dBA.
Typical prior art nozzles, as used for particle blast cleaning apparatuses, have been measured to emit noise as high as 130 dBA at the operator's position. In particular, the nozzle disclosed by U S. Pat. No. 4,038,786 has been documented to emit noise in the range of 127 dBA. Since the dB scale is logarithymic, a change of three decibels represents a doubling of the sound pressure level. Thus, the difference between the OSHA 120 dBA threshold level of pain and the 127 dBA of the typical prior art nozzle, represents over a fourfold increase in the sound pressure level. When compared to the 90 dBA OSHA limit, this difference is an increase of over 4,000 times the sound pressure level of 90 dBA. When compared to the 85 dBA standard of industry and the military, the 127 dBA level of the typical prior art nozzle has a sound pressure level more than 16,000 times this standard.
As is obvious, the sound emitted by supersonic nozzles must be reduced to as low a level as possible to permit safe continuous operation. While ear protection is available, such protective devices attenuate the noise only in the range of 20 to 25 dBA. This would result in a sound pressure level of the typical 127 dBA prior art nozzle of as low as a 102 dBA at the operator's position. While this would drastically reduce the sound pressure level experienced by the operator, it would still remain above the OSHA, industry, and military limits for eight hours of exposure. Furthermore, in a factory situation, it is unrealistic to require workers in nearby areas who are not involved with the supersonic nozzle use to wear such ear protection. The ultimate goal and solution is to use a supersonic nozzle which has a sound pressure level low enough that an operator wearing approved ear protection devices is subjected to less than 85 dBA. Such a nozzle would have to have a sound pressure level of less than 115 dBA at the operator position. The 115 dBA level would also be acceptable to workers without ear protection devices who are more than 15 feet from the operating nozzle.